This invention is directed to a heating control system for automatically controlling the temperature within a space, and in particular to a closed loop electronic heating control system having a thermistor as a linearly variable temperature sensor defining an open loop input for controlling the set rate at which heat is supplied to the space.
Heretofore, heating control systems for automatically controlling the temperature within a space have included resistance bridge circuits for controlling the rate at which heat is supplied to a space. Such systems usually include a heat balancer and one or two additional temperature sensors. A heat balancer includes a pair of temperature responsive resistance elements and a heating steam coil. A first temperature responsive resistance element is positioned in the path of the air flowing to the coil and the further temperature resistance element is positioned in the path of air flowing from the coil. The difference in the air temperatures, as represented by difference in the resistances of the respective temperature sensitive elements, constitutes an accurate indication of the amount of heat being added to the air passing through the heat convector unit. In addition to the temperature responsive resistance elements in the heat balancer, a temperature responsive resistance thermostat, for measuring outdoor and/or indoor temperatures is included in a balanced bridge circuit with the temperature responsive resistance elements of the heat balancer to produce control signals to be applied to a control valve in order to control the rate at which heat is supplied to the space.
The balanced bridge circuit formed by the respective temperature responsive resistance elements is coupled to the control valve motor and is maintained in a balanced condition by being coupled to the control valve motor, to thereby define a closed loop control system. Such a system is illustrated and described in U.S. Pat. No. 3,054,562 (Werts), and has been found to be less than completely satisfactory.